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Chapter 4: Application Layer Network Basics Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 1 Chapter 4 Objectives Explain how the functions of the application layer, session layer, and presentation layer work together to provide network services to end user applications. Describe how common application layer protocols interact with end user applications. Describe common application layer protocols that provide Internet services to end-users, including World Wide Web services and email. Describe application layer protocols that provide IP addressing services, including DNS and DHCP. Describe the features and operation of well-known application layer protocols that allow for file sharing services, including: FTP, File Sharing Services, SMB protocol. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 2 Chapter 4 4.1 Application Layer Protocols 4.2 Well-Known Application Layer Protocols and Service 4.3 Summary Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 3 4.1 Application Layer Protocols The Application Layer provides the interface to the network. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 4 Application Session and Presentation OSI and TCP/IP Models Revisited The key parallels are in the transport and network layer. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 5 Application Session and Presentation Application Layer Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 6 Application, Session and Presentation Presentation and Session Layers Presentation layer has three primary functions: Coding and conversion of application layer data Compression of the data Encryption of the data for transmission and the decryption of data upon receipt by the destination Session Layer Functions create and maintain dialogs between source and destination applications Handles the exchange of information to initiate dialogs, keep them active, and to restart sessions Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 7 Application Session and Presentation Presentation and Session Layer Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 8 Application, Session and Presentation TCP/IP Application Layer Protocols Domain Name Service Protocol (DNS) - used to resolve Internet names to IP addresses Telnet - a terminal emulation protocol used to provide remote access to servers and networking devices Bootstrap Protocol (BOOTP) - a precursor to the DHCP protocol, a network protocol used to obtain IP address information during bootup Dynamic Host Control Protocol (DHCP) - used to assign an IP address, subnet mask, default gateway and DNS server to a host Hypertext Transfer Protocol (HTTP) - used to transfer files that make up the Web pages of the World Wide Web Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 9 Application, Session and Presentation TCP/IP Application Layer Protocols File Transfer Protocol (FTP) - used for interactive file transfer between systems Trivial File Transfer Protocol (TFTP) - used for connectionless active file transfer Simple Mail Transfer Protocol (SMTP) - used for the transfer of mail messages and attachments Post Office Protocol (POP) - used by email clients to retrieve email from a remote server Internet Message Access Protocol (IMAP) – another protocol for email retrieval Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 10 Application, Session and Presentation Services at the Application Layer Within the application layer, there are two forms of software programs or processes that provide access to the network: Network-Aware Applications End-user applications - software programs used by people to communicate over the network Implement the application layer protocols and communicate directly with the lower layers Email clients and web browsers are examples Application Layer Services Need the assistance of application layer services to use network resources Example - file transfer or network print spooling Different types of data require different network services 11 Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential Application, Session and Presentation Services at the Application Layer Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 12 Application, Session and Presentation Applications Interface with People and Other Applications Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 13 Application, Session and Presentation Applications Interface with People and Other Applications Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 14 How Application Protocols Interact with End-User Applications Peer-to-Peer Networks Both devices are considered equal in the communication The roles of client and server are set on a per request basis. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 15 How Application Protocols Interact with End-User Applications Peer-to-Peer Applications Client and server in the same communication Both can initiate a communication and are considered equal in the communication process Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 16 How Application Protocols Interact with End-User Applications Common P2P Applications With P2P applications, each computer in the network running the application can act as a client or a server for the other computers in the network running the application Common P2P applications include: • eDonkey • eMule • Shareaza • BitTorrent • Bitcoin • LionShare Some P2P applications are based on the Gnutella protocol which enables people to share files on their hard disks with others Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 17 How Application Protocols Interact with End-User Applications Client-Server Model Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 18 How Application Protocols Interact with End-User Applications Client-Server Model Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 19 4.2 Well-Known Application Layer Protocols and Services Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 20 Everyday Application Layer Protocols Application Layer Protocols Revisited Three application layer protocols that are involved in everyday work or play are: Hypertext Transfer Protocol (HTTP) • browse the web Simple Mail Transfer Protocol (SMTP) • used to enable users to send email Post Office Protocol (POP) • used to enable users to receive email Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 21 Everyday Application Layer Protocols Hypertext Transfer Protocol / Markup Language Example URL: http://www.cisco.com/index.html First, the browser interprets the three parts of the URL: 1. http (the protocol or scheme) 2. www.cisco.com (the server name) 3. index.html (the specific file name requested) Browser checks with a name server to convert www.cisco.com into a numeric address Using the HTTP protocol requirements sends a GET request to the server and asks for the file index.html Server sends the HTML code for this web page Browser deciphers the HTML code and formats the page Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 22 Everyday Application Layer Protocols HTTP and HTTPS Presentation_ID Developed to publish and retrieve HTML pages Used for data transfer Specifies a request/response protocol Three common message types are GET, POST, and PUT GET is a client request for data POST and PUT are used to send messages that upload data to the web server 23 © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential Everyday Application Layer Protocols SMTP and POP Presentation_ID Typically use an application called a Mail User Agent (email client) Allows messages to be sent Places messages that have been replaced into the client's mailbox SMTP - Send email from either a client or a server POP - Receive email messages from an email server IMAP - Internet Message Access Protocol Email client provides the functionality of both protocols within one application © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 24 Common Application Layer Protocols SMTP and POP Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 25 Everyday Application Layer Protocols SMTP and POP (Continued) Presentation_ID Operates two processes: MTA and MDA MTA is used to forward email Addressed to a user whose mailbox is on the local server, mail is passed to the MDA Mail is for a user not on the local server, the MTA routes the email to the MTA on the appropriate server © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 26 Everyday Application Layer Protocols SMTP and POP (Continued) MDA accepts a piece of email from MTA and performs the actual delivery MDA receives all the inbound mail from the MTA and places it into mailboxes MDA can also resolve final delivery issues, such as virus scanning, spam filtering, and return-receipt handling Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 27 Everyday Application Layer Protocols SMTP and POP (Continued) Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 28 Providing IP Addressing Services Domain Name Service A human legible name is resolved to its numeric network device address by the DNS protocol Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 29 Providing IP Addressing Services Domain Name Service A human legible name is resolved to its numeric network device address by the DNS protocol Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 30 Providing IP Addressing Services DNS Message Format DNS server stores different types of resource records used to resolve names Contain the name, address, and type of record Record types are: A - an end device address NS - an authoritative name server CNAME - the canonical name for an alias; used when multiple services have the single network address but each service has its own entry in DNS MX - mail exchange record; maps a domain name to a list of mail exchange servers Unable to resolve the name using its stored records, contacts other servers Server temporarily stores the numbered address that matches the name in cache memory Windows ipconfig /displaydns displays all cached DNS Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 31 Providing IP Addressing Services DNS Hierarchy Examples toplevel domains: .au Australia .co Colombia .com business or industry .jp Japan .org non-profit organization Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 32 Providing IP Addressing Services nslookup Operating system utility called nslookup allows the user to manually query the name servers to resolve a given host name Utility can be used to troubleshoot name resolution issues and to verify the current status of the name servers Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 33 Providing IP Addressing Services Dynamic Host Configuration Protocol DHCP allows a host to obtain an IP address dynamically DHCP server is contacted and address requested - chooses address from a configured range of addresses called a pool and “leases” it to the host for a set period DHCP used for general purpose hosts such as end user devices, and static addressing is used for network devices such as gateways, switches, servers and printers Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 34 Providing IP Addressing Services DHCP Operation Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 35 Providing File Sharing Services File Transfer Protocol FTP allow data transfers between a client and a server FTP client is an application that runs on a computer that is used to push and pull data from a server running an FTP daemon To successfully transfer data, FTP requires two connections between the client and the server, one for commands and replies, the other for the actual file transfer Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 36 Providing File Sharing Services Server Message Block Clients establish a long term connection to servers After the connection is established, the user can access the resources on the server as if the resource is local to the client host Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 37 Providing File Sharing Services Server Message Block Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 38 Application Layer Summary Applications are computer programs with which the user interacts and which initiate the data transfer process at the user’s request. Services are background programs that provide the connection between the application layer and the lower layers of the networking model. Protocols provide a structure of agreed-upon rules and processes that ensure services running on one particular device can send and receive data from a range of different network devices. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 39 Application Layer Summary HTTP supports the delivery of web pages to end devices SMTP, POP, and IMAP support sending and receiving email SMB and FTP enable users to share files P2P applications make it easier for consumers to seamlessly share media DNS resolves the human legible names used to refer to network resources into numeric addresses usable by the network All of these elements work together, at the application layer. The application layer enables users to work and play over the Internet. Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 40 Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 41